The effects of oxygen toxicity on the newborn lung are largely unknown. The objectives of this research proposal are to show that: (1) Increased concentrations of oxygen at normal atmospheric pressures induce acute and chronic inhibition of DNA synthesis in the newborn mouse lung in vivo. (2) Such inhibition, both acute and chronic, is a function of the increased oxygen concentration and the duration of exposure. (3 The bronchiolar mucosal cell which shows injury and regeneration plays a major role in such damage. The study involves paired littermate control and experimental newborn mice maintained in controlled environment chambers in either an air or increased oxygen atmosphere, tritiated thymidine uptake into lung DNA, subsequent DNA extraction and analysis with liquid scintillation determination of the tritiated thymidine uptake, and autoradiography as well as light and electron microscopy.